The invention concerns a steering system for a vehicle with a first steering arrangement, particularly a wheel steering, which has a first sensor, with a second steering arrangement, particularly an articulated steering, which has a second sensor, and with a steering unit.
A conventional steering system is shown in U.S. Pat. No. 4,565,257. The vehicle shown there can both be steered in that the wheels are turned in relation to a first vehicle part and in that the first vehicle part is turned in relation to a second vehicle part, namely around a converting kit that is arranged approximately in the middle of the vehicle. The driver can select, which of the two steering forms he uses. When the wheels are turned in the same direction, in which also the vehicle is articulated, very narrow curve radii can be driven. When the wheels are turned in the opposite direction as the articulation of the vehicle, the vehicle can be driven in parallel to a predetermined line with the shovel in a slanting position.
A similar vehicle is shown in U.S. Pat. No. 4,802,545. There the wheels, which are arranged on a rotary stool, can be steered in relation to a vehicle part, while this vehicle part can be articulated, that is, turned, in relation to another vehicle part. The change-over between the two steering forms is made via a hydraulically operated selection valve.
DE 35 43 054 Al shows a hydraulic steering for vehicles with an articulated frame, on which also wheels are steered. The steering takes place in accordance with a predetermined schedule, which foresees that during steering, first the wheel steering is activated until the end stop, and only then the working cylinder for the articulated frame is activated, whereas for the return steering the working cylinder of the articulated frame is disconnected first, after which the wheels are straightened.
Changing over between the two steering forms that must be made by the driver in the two references mentioned first, means an additional operation, which must be performed by the driver. In the reference mentioned last, the individual steering movements can only be made one after the other.
Therefore, a principal object of this invention is to increase steering comfort of the vehicle for the operator.
The steering system of this invention has a steering unit that is fixedly connected to a first steering arrangement, and a second steering arrangement is connected to a follow-up arrangement, which comprises a first sensor, a control device and a valve arrangement.
Immediately, the driver basically activates only the first steering arrangement. The second steering arrangement follows the first steering arrangement in accordance with the xe2x80x9cmaster/slavexe2x80x9d principle. By means of the first sensor, the position of the first steering arrangement is monitored practically continuously, and passed on to the control device. The control device then controls a valve arrangement, which supplies the second steering arrangement with hydraulic fluid with the corresponding pressure and the corresponding direction. Thus, the two steering arrangements work together or simultaneously, so that the advantages of both steering arrangements can be utilised, without requiring any additional operations.
The follow-up arrangement comprises the second sensor. Then, the second steering arrangement is fixed in a control circuit, whose desired value is predetermined by the first steering arrangement. This improves the quality, with which the second steering arrangement can follow the first steering arrangement.
The valve arrangement is made as a proportional valve. Thus, it is particularly simple to have the second steering arrangement perform a proportional following of the movement of the first steering arrangement. Accordingly, the second steering arrangement will reach its end position, when the first steering arrangement is also in this position.
A supply device is common for both steering arrangements. The supply device has a priority device, which gives the second steering arrangement a higher priority than the first steering arrangement. This embodiment is particularly advantageous, when the second steering arrangement has a larger cylinder displacement than the first one. In this case, it must always be possible to supply sufficient hydraulic fluid to the articulated steering to enable it to follow the wheel steering.